Transcriptional regulatory factors are generally found to contain multiple domains, which provide different functions that only become apparent in the context of a specific promoter. HMG-1 enhances the binding affinity of hTBP for TATA and significantly increases the on-rate of binding. Stable complexation requires both the acidic C-terminus of HMG-1 to the Q-tract in the N-terminus of hTBP. Definition of the specific domains responsible for the interactions HMG-1ITBP interaction provides the first basis for a deeper understanding that suggests a specific mechanism by which HMG-1 functions as a transcriptional repressor by physically interfering with subsequent TFIIB binding. On the other hand, HMGI enhances the binding of a select group of transcriptional activators. These "HMG-1-sensitive~' activators exhibit increased binding affinity in the presence of HMG-1, which correlates with stimulation of activated transcription. The individual A-&/or B-boxes of HMG-1 appear similar in achieving the latter effect. That is, these domains appear to act similarly as coactivators. We will begin to define the thermodynamics and kinetics associated with coactivator, HMG-1, interactions in a model system involving regulatory factor, Oct-1, binding to the OCTA site, in promoters with and without bound TBPITATA. These data will further provide the basis that can lead to defining a possible role and a comprehensive mechanism by which the modular, multiple domain HMG-1 protein can serve as a coactivator for "HMG-1-sensitive" activators, such as Oct-1. We will determine the effect of HMG-1 on Oct1/OCTA binding and try to determine whether the HMG-1/TBP/TATA complex plays a significant role in effectively recruiting the activator. The model will be tested using HMG-1 and the HMG-1 (A-B) di-domain proteins, together with TBP and cTBP, in the context of the RNA poI III promoter in the histone H2B promoter and, if time permits, with the immunoglobulin k light chain promoter. Preliminary studies will be aimed at extending our finding of HMG-1 complexation with TBP/TATA to the influence of HMG-1 on HeLa cell TFIID, with the aim of determining its influence on TFIID promoter binding. Finally, another outcome of our characterization of the HMG-1/TBP/TATA complex is the prediction that HMG-I will act as a repressor of RNA pol Ill transcription at the U6 promoter by specific interference with SNAPc/hTBP complex formation. HMG-1 and SNAP bind essentially to the same region in the N-terminus of hTBP. Binding studies will be done to determine if HMG-1 interferes with this nucleation event and if the effect correlates with the extent of RNA poI III transcription. The inclusion of two classes of RNA polymerases that are regulated by HMG-1 adds to its importance as a regulatory transcription factor.